Device for anchoring a drill string in a borehole

ABSTRACT

A device for anchoring a drill string in a borehole formed in an earth formation is provided. The device comprises an anchoring member connected to the drill string and being movable between a retracted position in which the anchoring member is retracted from the borehole wall and an extended position in which the anchoring member is extended against the borehole wall so as to anchor the drill string to the borehole wall, and an activating member operable to move the anchoring member from the extended position to the retracted position by the action of pressure of drilling fluid present in the borehole.

[0001] The present invention relates to a device for anchoring a drillstring in a borehole formed in an earth formation. In drilling deepboreholes or drilling boreholes at high inclination angles, it is acommon problem to provide sufficient forward thrust to the drill bit.Frictional forces between the drill string an the borehole wall largelyreduce the effective weight of the drill string providing forward thrustto the drill bit.

[0002] International patent application WO 99/09290 discloses a drillstring system provided with a thruster to thrust the drill bit inforward direction, and an anchoring device including radially extendiblegrippers with actuator pistons to anchor the drill string to theborehole wall during activation of the thruster.

[0003] A problem of the known anchoring device is that a separateactuating means is required to bring the pistons (and thereby also thegrippers) back to their retracted position after drilling of a furtherborehole section.

[0004] It is an object of the invention to provide an improved anchoringdevice which overcomes the drawbacks of the prior art anchoring device.

[0005] In accordance with the invention there is provided a device foranchoring a drill string in a borehole formed in an earth formation,comprising:

[0006] an anchoring member connected to the drill string and beingmovable between a retracted position in which the anchoring member isretracted from the borehole wall and an extended position in which theanchoring member is extended against the borehole wall so as to anchorthe drill string to the borehole wall; and

[0007] an activating member operable to move the anchoring member fromthe extended position to the retracted position by the action ofpressure of drilling fluid present in the borehole.

[0008] It is thereby achieved that the anchoring member is brought backto its retracted position by the pressure of drilling fluid in theborehole acting on the activating member, thereby obviating the need fora separate actuating means.

[0009] Suitably the activating member is arranged to move the anchoringmember from the extended position to the retracted position by theaction of said pressure of drilling fluid being present in an annularspace between the drill string and the borehole wall.

[0010] It is preferred that the activating member includes apiston/cylinder assembly arranged to move the anchoring member from theextended position to the retracted position upon a relative axialmovement between the piston and the cylinder by the action of saidpressure of drilling fluid acting on the piston.

[0011] The invention will be described hereinafter in more detail and byway of example, with reference to the accompanying drawings in which:

[0012]FIG. 1 schematically shows a drilling assembly in which the deviceof the invention is applied;

[0013]FIG. 2 schematically shows an embodiment of a hydraulic controlsystem for use in the device of the invention; and

[0014]FIG. 3 schematically shows an alternative embodiment of ahydraulic control system for use in the device of the invention.

[0015] In the Figures, like reference numerals relate to likecomponents.

[0016] Referring to FIG. 1 there is shown a drill string 1 extendinginto a borehole 2 formed in an earth formation 3, with an annular space4 between the drill string and the borehole wall 5. The drill string hasan upper part 6 and a lower part 8 provided with a drill bit 9, wherebythe parts 6, 8 are interconnected by a hydraulically activatedtelescoping thruster 10 capable of thrusting the lower drill string part8 in the direction of the borehole bottom. The upper drill string part 6and the lower drill string part 8 are provided with respective sets ofanchoring members 12 (e.g. three) in the form of pads regularly spacedalong the drill string circumference. Each pad 12 is connected to therespective drill string part 6, 8 in a manner that the pad 12 is movablebetween a retracted position in which the pad 12 is retracted from theborehole wall 5 and an extended position in which the pad 12 is extendedagainst the borehole wall 5 so as to anchor the respective drill stringpart 6, 8 to the borehole wall 5. The drill string is internallyprovided with a control system (schematically shown in FIG. 2) forcontrolling movement of each pad 12 between its retracted position andits extended position.

[0017] Referring further to FIG. 2 there is shown a schematicrepresentation of the control system for controlling movement of thepads 12, which comprises a hydraulic circuit 20 including a firstpiston/cylinder assembly 22 with a piston 24 which sealingly extendsinto a cylinder 26 and which is axially movable relative to the cylinder26 in outward direction A and inward direction B. The control systemfurther comprises a second piston/cylinder assembly 28 with a piston 30which sealingly extends into a cylinder 32 and which is axially movablerelative to the cylinder 32 in opposite directions C and D. The piston30 is provided with an auxiliary piston 30 a which sealingly extendsinto an auxiliary cylinder 32 a connected to the cylinder 32. A fluidchamber 32 b is defined in the auxiliary cylinder 32 a between theauxiliary piston 30 a and an end wall 32 c of the auxiliary cylinder 32a. The auxiliary piston 30 a is of smaller outer diameter than thepiston 30, and the auxiliary cylinder 30 a is of smaller inner diameterthan the cylinder 32.

[0018] The piston 24 has an outer end surface 34 which is subjected to apressure P of drilling fluid present in the annular space 4, and aninner end surface 36 subjected to a pressure of hydraulic fluid presentin a fluid chamber 37 of the cylinder 26. The piston 24 is connected byconnecting means (not shown) to the pads 12 in a manner that the piston24 induces the pads 12 to move to their extended position upon movementof the piston 24 in outward direction A, and that the piston 24 inducesthe pads 12 to move to their retracted position upon movement of thepiston 24 in inward direction B.

[0019] The piston 30 has a first end surface 40 in fluid communicationwith a low pressure chamber 42 of the second assembly 28 and a secondend surface 44 subjected to a pressure of hydraulic fluid present in afluid chamber 45 of the cylinder 32. The low pressure chamber 42contains a gas at low pressure or, ideally, is vacuum. The chamber 37 isin fluid communication with the fluid chamber 45 via conduits 46 a, 46 band a three-way valve 47.

[0020] The hydraulic circuit 20 furthermore comprises a hydraulic fluidpump 50 having an inlet 52 in fluid communication with a hydraulic fluidreservoir 54 via a conduit 56, and an outlet 58 in fluid communicationwith the chamber 37 via a conduit 60 provided with a valve 61. Theoutlet 58 is furthermore in fluid communication with the first fluidchamber 32 b via a conduit 62, a three-way valve 63 and a conduit 64.The fluid reservoir 54 is in fluid communication with the conduit 46 viaa conduit 66 and the three-way valve 47, and with the conduit 64 via aconduit 69 and the three-way valve 63. Fluid reservoir 54 is pressurecompensated by means of a piston 70 provided to the reservoir 54, whichpiston 70 transfers the drilling fluid pressure P to the hydraulic fluidpresent in fluid reservoir 54. Furthermore low pressure chamber 42 isconnected via a conduit 71 to conduit 69, which conduit 71 is providedwith a one-way valve 72 allowing fluid to flow only from chamber 42 toconduit 69.

[0021] The piston 24 is connected by connecting means (not shown) to thepads 12 in a manner that the piston 24 induces the pads 12 to move totheir extended position upon movement of the piston 24 in outwarddirection A, and that the piston 24 induces the pads 12 to move to theirretracted position upon movement of the piston 24 in inward direction B.

[0022] Referring to FIG. 3 there is shown a schematic representation ofthe alternative control system for controlling movement of the pads 12.The alternative control system comprises a hydraulic circuit 80 which issimilar to the control circuit 20, except that in the hydraulic circuit80 a third piston/cylinder assembly 82 replaces the secondpiston/cylinder assembly 28 referred to hereinbefore. The thirdpiston/cylinder assembly 82 includes a piston 84 which sealingly extendsinto a cylinder 86 and which is axially movable relative to the cylinder86 in opposite directions E and F. The piston 84 is provided with anauxiliary piston 84 a which extends into an auxiliary cylinder 86 aconnected to the cylinder 86. The piston 84 has an end surface 90 at theside of the auxiliary piston 84 a and an end surface 92 opposite the endsurface 90. The auxiliary piston 84 a has an end surface 94. A firstfluid chamber 96 is defined in the cylinder 86, between the end surface92 and an end wall 98 of the cylinder 86. A second fluid chamber 100 isdefined in the cylinder 86, between the end surface 90 and the other endwall 102 of the cylinder 86. A third fluid chamber 104 is defined in theauxiliary cylinder 86 a, between the end surface 94 and an end wall 106of the auxiliary cylinder 86 a. The first fluid chamber 96 is in fluidcommunication with the outlet 58 of the pump 50 via the three-way valve63. The second fluid chamber 100 is in fluid communication with theconduit 62 via conduits 110, 111 and a three-way valve 112, and with thehydraulic fluid reservoir 54 via conduits 110, 113 and the three-wayvalve 112.

[0023] In the following description normal use of the device accordingto the invention is described for activation and de-activation of asingle pad 12 of upper drill string part 6, with the understanding thatactivation and de-activation of the other pads 12 occurs in a similarmanner.

[0024] During normal use of the device with the control system of FIG.2, the valve 61 is opened and the three-way valve 47 is opened such thatfluid can flow via conduits 46 a, 46 b into fluid chamber 45. Three-wayvalve 63 is opened such that fluid can flow from chamber 32 b viaconduits 64, 69 into reservoir 54. Next the pump 50 is operated to pumphydraulic fluid from the fluid reservoir 54 into the fluid chamber 37 ofcylinder 26 and into chamber 45 of cylinder 32. As a result piston 24moves in outward direction A and thereby moves the pads 12 against theborehole wall 5 so as to anchor the upper drill string part 6 in theborehole, and piston 30 and auxiliary piston 30 a move in direction Cthereby discharging any hydraulic fluid which might have leaked into lowpressure chamber 42, to fluid reservoir 54 via conduits 71, 69 andone-way valve 72. Then three-way valve 47 is opened such that fluid canflow via conduits 46 b and 66 into reservoir 54, and three-way valve 63is opened such that hydraulic fluid flows from outlet 58 of pump 50 viaconduits 62, 64 into fluid chamber 32 b of cylinder 32 a thereby pushingpiston 30 and auxiliary piston 30 a in direction D. As a result a verylow gas pressure (or preferably vacuum) is created in chamber 42. Theborehole 2 is then further drilled by simultaneously rotating the drillbit 9 and inducing the thruster 10 to thrust the drill bit 9 against theborehole bottom. After drilling of a further borehole section iscompleted, rotation of the drill bit 9 and operation of the pump 50 isstopped, whereafter the valve 61 is closed. The valve 47 is then openedso as to bring conduit 46 a in fluid communication with conduit 46 b,and the valve 63 is opened so as to bring chamber 32 b in communicationwith reservoir 54 via conduits 64, 69. As a result the drilling fluidpressure P moves the piston 24 in inward direction B whereby hydraulicfluid flows from fluid chamber 37 via conduits 46 a, 46 b into fluidchamber 45, and from chamber 32 b into reservoir 54, and the piston 30and auxiliary piston 30 a move in the direction C by virtue of thepressure in fluid chamber 45 being larger than the pressure (or vacuum)in low pressure chamber 42. The pads 12 are retracted from the boreholewall 5 by the inward movement of the piston 24.

[0025] In a next step the upper drill string part 6 is moved furtherdownward in the borehole. Then the valve 63 is opened so as to bringconduit 62 in fluid communication with conduit 64, and the valve 47 isopened so as to bring conduit 46 b in fluid communication with conduit66. The pump 50 is then operated to pump hydraulic fluid from reservoir54 via conduits 62, 64 into the fluid chamber 32 b thereby pushingauxiliary piston 30 a and piston 30 in direction D. Hydraulic fluidpresent in fluid chamber 45 flows thereby via conduits 46 b and 66 intoreservoir 54. Thereafter the pads 12 are again extended against theborehole wall 5 in the manner described hereinbefore, and a yet furtherborehole section is drilled.

[0026] Normal use of the device with the control system of FIG. 3 issubstantially similar to normal use of the device with the controlsystem of FIG. 2. Valve 61 is opened, and the three-way valve 47 isopened such that fluid can flow via conduits 46 b and 66 into reservoir54. Three-way valve 112 is opened such that fluid from chamber 100 canbe discharged via conduits 110 and 113 into reservoir 54. Three-wayvalve 63 is opened such that chamber 96 is hydraulically connected viaconduits 64 and 62 to the pump-outlet 58. Then the pump 50 is operatedto pump hydraulic fluid from the fluid reservoir 54 into the fluidchamber 37 of cylinder 26 and into chamber 96 of cylinder 82. As aresult the piston 24 moves in outward direction A and thereby moves thepads 12 against the borehole wall 5 so as to anchor the upper drillstring part 6 to the borehole wall 5, and piston 84 and auxiliary piston84 a move in direction F thereby discharging hydraulic fluid from fluidchambers 100 and 104 into reservoir 54. The borehole 2 is then furtherdrilled by simultaneously rotating the drill bit 9 and inducing thethruster 10 to thrust the drill bit 9 against the borehole bottom. Afterdrilling of a further borehole section is completed, rotation of thedrill bit 9 is stopped. Valve 61 is closed, and valve 47 is then openedso as to bring chamber 37 in fluid communication with chamber 104 viaconduit 46 a and conduit 46 b. Three-way valve 63 is opened such thatfluid can be discharged from chamber 96 via conduits 64 and 69 intoreservoir 54. Three-way valve 112 is opened such that chamber 100 is influid communication with pump-outlet 58 via conduits 110, 111 and 62. Bypumping hydraulic fluid into chamber 100, piston 84 and auxiliary piston84 a are pushed in direction E. As a result the drilling fluid pressureP moves the piston 24 in inward direction B whereby hydraulic fluidflows from fluid chamber 37 via conduits 46 a, 46 b into fluid chamber104, and from chamber 96 into reservoir 54. The pads 12 are. retractedfrom the borehole wall 5 by the inward movement of the piston 24.

[0027] Instead of opening the valve 61 and closing the valve 47 beforeoperating the pump 50 to move the piston 24 in outward direction A, thefollowing alternative procedure can suitably be followed. The valve 61is closed, the valve 63 is opened so as to provide fluid communicationbetween conduits 62, 64, and the valve 47 is opened so as to providefluid communication between conduits 46 a, 46 b. The valve 112 is openedso as to provide fluid communication between chamber 100 and reservoir54 via conduits 110, 113. The pump 50 is then operated so as to pumphydraulic fluid via conduits 62, 64 into the first fluid chamber 96,with the result that the piston 84 and auxiliary piston 84 a move indirection F. Hydraulic fluid is thereby displaced from the third fluidchamber 104 via conduits 46 b, 46 a into the fluid chamber 37 ofcylinder 26, resulting in movement of the piston 26 in outward directionA. The alternative procedure has the advantage that the fluid pressurein fluid chamber 37 is substantially increased during pumping due to thepiston 84 being of larger diameter than auxiliary piston 84 a.

[0028] During a suitable drilling procedure, the pads 12 of the lowerdrill string part 8 are extended against the borehole wall only duringperiods of time that the pads 12 of the upper drill string member areretracted from the borehole wall in order to provide a reactive torqueto the lower drill string part in case of continued rotation of thedrill bit which is driven by a downhole motor.

[0029] Instead of, or in addition to, moving the piston 30, 84 of thesecond or third piston/cylinder assembly 28, 82 in respective directionsC or E by the action of a fluid pressure difference across the piston30, 84, such movement can be achieved by the action of a spring arrangedin a suitable manner in the second or third piston/cylinder assembly.

[0030] In the above description the anchoring member and thecorresponding activating member are described as separate components.Alternatively, the anchoring member and the corresponding activatingmember can be integrally formed as a single component.

1. A device for anchoring a drill string in a borehole formed in anearth formation, comprising: an anchoring member connected to the drillstring and being movable between a retracted position in which theanchoring member is retracted from the borehole wall and an extendedposition in which the anchoring member is extended against the boreholewall so as to anchor the drill string to the borehole wall; and anactivating member operable to move the anchoring member from theextended position to the retracted position by the action of pressure ofdrilling fluid present in the borehole.
 2. The device of claim 1,wherein the activating member is arranged to move the anchoring memberfrom the extended position to the retracted position by the action ofsaid pressure of drilling fluid being present in an annular spacebetween the drill string and the borehole wall.
 3. The device of claim 1or 2, wherein the activating member includes a piston/cylinder assemblyarranged to move the anchoring member from the extended position to theretracted position upon a relative axial movement between the piston andthe cylinder by the action of said pressure of drilling fluid acting onthe piston.
 4. The device of claim 3, further comprising a controlsystem for selectively inducing a force to the piston which counter-actssaid relative axial movement between the piston and the cylinder therebycounter-acting movement of the anchoring member from the extendedposition to the retracted position.
 5. The device of claim 4, whereinthe control system includes a hydraulic circuit arranged to induce saidforce to the piston by exertion of a hydraulic fluid pressure to thepiston.
 6. The device of claim 5, wherein the hydraulic circuit isprovided with pressure reduction means for selectively reducing thehydraulic fluid pressure exerted to the piston.
 7. The device of claim6, wherein said piston/cylinder assembly forms a first piston/cylinderassembly, and wherein the pressure reduction means includes a secondpiston/cylinder assembly in fluid communication with the firstpiston/cylinder assembly via a hydraulic fluid conduit, said secondpiston/cylinder assembly being arranged to reduce the hydraulic fluidpressure exerted to the piston of the first assembly by a relative axialmovement between the piston and cylinder of the second assembly.
 8. Thedevice of claim 7, wherein the hydraulic circuit is provided withpowering means for inducing a pressure difference across the piston ofthe second piston/cylinder assembly so as to induce said relative axialmovement between the piston and cylinder of the second assembly.
 9. Thedevice of claim 8, wherein the powering means includes a low pressurechamber in fluid communication with an end surface of the piston of thesecond assembly.
 10. The device of claim 8 or 9, wherein the poweringmeans includes a high pressure chamber in fluid communication with thepiston of the second assembly and a pump for pumping hydraulic fluidinto the high pressure chamber.
 11. The device of claim 10, wherein thepump is arranged to selectively pump hydraulic fluid to the firstpiston/cylinder assembly so as to induce the piston of the firstpiston/cylinder assembly to move the anchoring member from the retractedto the extended position.
 12. The device substantially as describedhereinbefore with reference to the drawings.